In a coating method for obtaining a transparent electroconductive film by coating an electroconductive ink on a substrate, used is an oxide filler such as indium-tin oxide (ITO), tin-antimony oxide (ATO), etc., as the electroconductive filler in the ink. Of these, ITO is the best, as having a low resistance value than ATO.
Of electroconductive inks, those having a smaller content of the electroconductive filler therein are more preferred. This is because the light absorption of the filler of an oxide is far larger than that of a transparent resin which is another component in the ink. Therefore, if a film having a low resistance value is obtained, using the least possible amount of the oxide filler relative to the resin, the light transmittance of the film may be improved.
Regarding the electroconductive filler of the kind, acicular (needle-like) or flaky ones are considered preferable. This is because smaller amounts of them may be used for obtaining films having a low resistance value, as compared with using spherical or granular electroconductive fillers, and the films are satisfactory with respect to the cost, the physical strength, the weather resistance, etc.
There are known a method of obtaining flaky oxides by freezing a colloid liquid of an inorganic oxide or hydroxide so as to make fine particles of the oxide or hydroxide precipitated among the solvent molecules of the colloid liquid followed by drying it so as to remove the solvent therefrom and, for the hydroxide, further roasting it to, finally obtain flaky oxides (see Japanese Patent Laid-Open No. 62-3003), and a method of obtaining tin oxide aciculae by heating and decomposing tin oxalate aciculae (see Japanese Patent Laid-Open No. 56-120519). However, powder of ITO aciculae having a high aspect ratio has not been obtained.
As conventional electroconductive pastes to be used for forming light-transmitting electrodes such as those in electroluminescence (EL) devices, etc., known are a paste that is obtained by dispersing an electroconductive filler comprising an ultra-fine powder of indium-tin oxide (ITO) in a solvent containing a resin dissolved therein, and a paste that is obtained by dispersing a powder of ITO flakes in a solvent containing a resin dissolved therein.
In organic dispersion-containing EL devices, the substrate has a light-emitting zinc sulfide (ZnS) layer formed thereon by screen-printing or blade-coating and has thereover a light-transmitting electroconductive film formed by coating an electroconductive paste thereon by screen-printing or the like.
In the electroconductive paste containing an ultra-fine powder of ITO as the electroconductive filler, the amount of the electroconductive filler must be much more than that of the resin so as to obtain the intended electroconductivity. It is said that the light-transmitting electroconductive film is desired to be thin, preferably having a thickness of approximately from 2 to 3 .mu.m, so as to obtain the necessary light transmittance.
The printed surface of the light-emitting zinc sulfide layer is often rough, since the particles of zinc sulfide therein have a size of several ten .mu.m. Therefore, when an electroconductive paste is coated thereover, the film of the electroconductive paste becomes uneven due to the roughness of the surface of the zinc sulfide layer to partly have a thickness of 1 .mu.m or less or a thickness of 5 .mu.m or more, and a uniform electroconductive film having a thickness of from 2 to 3 .mu.m cannot be formed on the whole surface of the light-emitting layer. The thin parts of the coated electroconductive film were often cracked to unfavorably increase the resistance of the film.
In the electroconductive paste that is prepared by dispersing a powder of ITO flakes in a solvent containing a resin dissolved therein, the content of the electroconductive filler may be smaller than that of the resin in order to obtain the intended electroconductivity. Therefore, even if it is coated at a thickness of 5 .mu.m or more, the coated film may have sufficient light transmittance. For this reason, the roughness of the surface of the zinc sulfide layer does not cause so much significant problem in this case. However, it is impossible to say that the resistance of the film is sufficient.
Of dispersion-containing EL devices, known are those that are produced by a high-temperature process using an inorganic binder such as glass or the like, at 500.degree. to 600.degree. C. For producing EL devices of this type, for example, a white enamel layer comprising BaTiO.sub.3, TiO.sub.2 or the like is formed on a low-carbon steel plate by baking it, and a fluorescent layer is formed thereon by baking a mixture comprising a transparent glass having a low melting point and a high dielectric constant and a fluorescent powder. In these devices, a NESA film that is formed by chemical vapor deposition (CVD) of a chloride is used as the light-transmitting electrode.
However, the NESA film has the following drawbacks:
(1) As this is formed by CVD, the electroconductive film is formed on the whole surface and it is difficult to pattern the film. PA1 (2) All the white enamel layer, the fluorescent layer and the transparent protective layer (which is formed on the light-transmitting electroconductive film) are formed by printing, for example, by screen-printing or the like, but only the NESA film is formed by CVD. Therefore, the NESA film is inconvenient for simplifying the production process. PA1 (1) A method for producing a first raw material to be used for producing a powder of indium-tin oxide aciculae, characterized in that an aqueous solution containing indium ions and nitrato ions is heated and concentrated to form a high-viscosity slurry and a powder of aciculae is separated from the slurry. PA1 (2) A method for producing a first raw material to be used for producing a powder of indium-tin oxide aciculae, characterized in that an aqueous solution containing indium ions and nitrato ions and also containing indium hydroxide and/or indium oxide is heated and concentrated to form a high-viscosity slurry and a powder of aciculae is separated from the slurry. PA1 (3) A preferred embodiment of the above-mentioned (1) or (2) for producing a first raw material to be used for producing a powder of indium-tin oxide aciculae, in which the slurry is mixed with a large amount of water or with a large amount of water containing an alkali component and subjected to solid-liquid separation. PA1 (4) A preferred embodiment of the above-mentioned (1) or (2) for producing a first raw material to be used for producing a powder of indium-tin oxide aciculae, in which the slurry is filtered to obtain a filtered cake and the cake is mixed with a large amount of water or with a large amount of water containing an alkali component and subjected to solid-liquid separation. PA1 (5) A method for producing a first raw material to be used for producing a powder of indium-tin oxide aciculae, characterized in that lithium nitrate or a nitrate containing lithium nitrate is added to an aqueous solution containing indium ions and nitrato ions, then the solution is heated and concentrated to form a high-viscosity slurry, and a powder of aciculae is separated from the slurry. PA1 (6) A method for producing a second raw material to be used for producing a powder of indium-tin oxide aciculae, characterized in that the first raw material for producing a powder of indium-tin oxide aciculae that has been produced according to the method of the above-mentioned (1) or (2) is reacted with an aqueous alkaline solution. PA1 (7) A powder of indium oxide aciculae for producing a powder of indium-tin oxide aciculae, which have a length of 5 .mu.m or more and a ratio of length to cross-sectional diameter of 5 or more. PA1 (8) A method for producing a powder of indium oxide aciculae to be used for producing a powder of indium-tin oxide aciculae, which have a length of 5 .mu.m or more and a ratio of length to cross of 5 or more; the method being characterized in that the first raw material for producing a powder of indium-tin oxide aciculae that has been produced according to anyone of the methods of the above-mentioned (1) to (5) or the second raw material for producing a powder of indium-tin oxide aciculae that has been produced according to the method of the above-mentioned (6) is calcined. PA1 (9) A method for producing a powder of indium-tin oxide aciculae, characterized in that an aqueous solution containing indium ions, nitrato ions and a tin compound capable of being converted into SnO.sub.2 by calcination is heated and concentrated to form a high-viscosity slurry, a first intermediate powder of aciculae is separated from the slurry, and the powder is then calcined. PA1 (10) A method for producing a powder of indium-tin oxide aciculae, characterized in that an aqueous solution containing indium ions, nitrato ions, indium hydroxide and/or indium oxide, and a tin compound capable of being converted into SnO.sub.2 by calcination is heated and concentrated to form a high-viscosity slurry, a first intermediate powder of aciculae is separated from the slurry, and the powder is then calcined. PA1 (11) A method for producing a powder of indium-tin oxide aciculae, characterized in that an aqueous solution containing indium ions, nitrato ions and an indium-tin oxide is heated and concentrated to form a high-viscosity slurry, a first intermediate powder of aciculae is separated from the slurry, and the powder is then calcined. PA1 (12) A preferred embodiment of anyone of the above-mentioned (9), (10) and (11) for producing a powder of indium-tin oxide aciculae, in which the slurry is mixed with a large amount of water or with a large amount of water containing an alkali component and subjected to solid-liquid separation. PA1 (13) A preferred embodiment of anyone of the above-mentioned (9), (10) and (11) for producing a powder of indium-tin oxide aciculae, in which the slurry is filtered to obtain a filtered cake and the cake is mixed with a large amount of water or with a large amount of water containing an alkali component and subjected to solid-liquid separation. PA1 (14) A preferred embodiment of anyone of the above-mentioned (9) to (13) for producing a powder of indium-tin oxide aciculae, in which the first intermediate powder of aciculae is reacted with an aqueous alkaline solution to obtain a second intermediate powder of aciculae and the second intermediate powder is calcined. PA1 (15) A preferred embodiment of anyone of the above-mentioned (9) to (14) for producing a powder of indium-tin oxide aciculae, in which the calcination is effected in an inert gas or in vacuum. PA1 (16) A method for producing a powder of indium-tin oxide aciculae, characterized in that an aqueous solution containing indium ions and nitrato ions is heated and concentrated to form a high-viscosity slurry, a first intermediate powder of aciculae is separated from the slurry, and the aciculae of the first intermediate powder are coated with a tin compound capable of being converted into tin dioxide by calcination and then calcined. PA1 (17) A method for producing a powder of indium-tin oxide aciculae, characterized in that an aqueous solution containing indium ions, nitrato ions, and indium hydroxide and/or indium oxide is heated and concentrated to form a high-viscosity slurry, a first intermediate powder of aciculae is separated from the slurry, and the aciculae of the first intermediate powder are coated with a tin compound capable of being converted into tin dioxide by calcination and then calcined. PA1 (18) A preferred embodiment of the above-mentioned (16) or (17) for producing a powder of indium-tin oxide aciculae, in which the slurry is mixed with a large amount of water or with a large amount of water containing an alkali component and subjected to solid-liquid separation. PA1 (19) A preferred embodiment of anyone of the above-mentioned (16) or (17) for producing a powder of indium-tin oxide aciculae, in which the slurry is filtered to obtain a filtered cake and the cake is mixed with a large amount of water or with a large amount of water containing an alkali component and subjected to solid-liquid separation. PA1 (20) A method for producing a powder of indium-tin oxide aciculae, characterized in that the first intermediate powder of aciculae that has been produced according to anyone of the methods of the above-mentioned (16) to (19) is reacted with an aqueous alkaline solution to obtain a second intermediate powder of aciculae, and the aciculae of the second intermediate powder are coated with a tin compound capable of being converted into tin dioxide by calcination and then calcined. PA1 (21) A method for producing a powder of indium-tin oxide aciculae, characterized in that the first intermediate powder of aciculae that has been produced according to anyone of the methods of the above-mentioned (16) to (19) or the second intermediate powder of aciculae that has been produced according to the method of the above-mentioned (20) is calcined to obtain a powder of indium oxide aciculae, and the aciculae of the indium oxide powder are coated with a tin compound capable of being converted into tin dioxide by calcination and then calcined. PA1 (22) A preferred embodiment of anyone of the above-mentioned (16) to (21) for producing a powder of indium-tin oxide aciculae, in which the calcination is effected in an inert gas or in vacuum. PA1 (23) An electroconductive paste containing a powder of indium-tin oxide aciculae having a length of 5 .mu.m or more and a ratio of length to cross-sectional diameter of 5 or more, in a resin along with its solvent. PA1 (24) A preferred embodiment of the electroconductive paste of the above-mentioned (23), in which the ratio by weight of the fine powder of indium-tin oxide aciculae to the resin is from 60:40 to 80:20. PA1 (25) An electroconductive paste for high-temperature baking, which contains a transparent glass frit, a solvent and a fine powder of indium-tin oxide aciculae having a length of 5 .mu.m or more and a ratio of the length to the cross-sectional diameter of 5 or more. PA1 (26) A preferred embodiment of the electroconductive paste of the above-mentioned (25), in which the ratio by volume of the fine powder of indium-tin oxide aciculae to the transparent glass frit is from 10:90 to 50:50. PA1 (27) A light-transmitting electroconductive film composed of a fine powder of indium-tin oxide aciculae and a resin, which is characterized in that the specific resistance of the film is 1.0 .OMEGA..cm or less and the content of the fine powder of indium-tin oxide aciculae in the film is 25% by volume or less. PA1 (28) A light-transmitting electroconductive film composed of a fine powder of indium-tin oxide aciculae and a glass, which is characterized in that the specific resistance of the film is 5.0 .OMEGA..cm or less.